1. Field of the Invention
The present invention relates to a multipole lens having multipolar elements, such as octupolar elements or dodecapole (twelve-pole) elements. The invention also relates to a charged-particle beam instrument fitted with such multipole lenses. Furthermore, the invention relates to a method of fabricating such a multipole lens.
2. Description of Related Art
In an observation system, such as a charged-particle beam instrument (e.g., a scanning electron microscope), aberration in the electron beam, i.e., charged-particle beam, is corrected when the beam is directed at a specimen in order to appropriately image the specimen. Within such an observation system, multipole lenses are mounted as an aberration corrector to correct the aberration in the electron beam.
One design of such multipole lenses is disclosed in Japanese Patent Laid-Open Application No. H2-230647. This known structure has plural (eight or twelve) polar elements which are supported by an annular holding member and by a yoke disposed outside the holding member. This multipole lens (multipolar elements) shown in FIG. 1 of the Japanese application has plural polar elements, a beam tube SR, and an annular yoke J made of an iron circuit and disposed outside the beam tube. Each of the polar elements consists of a mounting rod PH and a polepiece PS coupled to the front end of the mounting rod. The beam tube is provided with hermetically sealed holes for passage of the mounting rods of the polar elements.
Each polar element is fabricated by screwing the front-end portion of each mounting rod into the polepiece or adhesively bonding or welding them together. The mounting rods and polepieces forming the polar elements are made of a magnetic material. The beam tube is made of an electrically insulating material. A metal coating is formed around each hermetically sealed hole.
The mounting rod of each polar element has a base-end portion firmly joined to the yoke. In particular, the mounting rod is inserted into the corresponding hole in the yoke via an insulator IS. Thus, the mounting rods are aligned and firmly coupled to the yoke.
The mounting rods are hermetically and rigidly mounted in the hermetic holes in the beam tube by the aforementioned welding via the metal coating. As a result, the weldments form a hermetically sealed body.
A coil is mounted on each mounting rod between the beam tube and yoke. The polepiece joined to the front end of each mounting rod is magnetically excited by passing electrical current through the coil. The base-end portions of the mounting rods are positioned in the holes of the yoke via the insulator. Electrical terminals are connected with the base-end portions, respectively, such that a voltage is supplied to them.
In the multipole lens of the construction as described above, the base-end portions of the polar elements of the mounting rods are inserted in the holes formed in the yoke and aligned. Under this condition, the base-end portions are firmly coupled to the yoke in the holes.
In this case, the base-end portions of the polar elements are firmly coupled in the holes formed in the yoke and so undesired external force may be applied to the base-end portions of the polar elements via the holes in the yoke during the fabrication step for joining the base-end portions to the yoke. If this force is applied, the polar elements may be deformed during the joining step or the shape of the polar elements may vary with time even where the polar elements are aligned at the beginning.
As a result, the polepieces at the front ends of the polar elements may deviate in position. A positional deviation along the optical axis of the electron beam produces a local field variation along the optical axis in the region where a magnetic or electric field is produced by the multipole lens. This local field variation is produced only at the beam entrance and exit portions of the region where the field is produced. Therefore, this local field variation will not create a great obstacle in correcting aberration in the electron beam. However, positional deviations of the polepieces in a direction perpendicular to the optical axis of the beam will create non-local field variations over the whole region through which the beam passes. These non-local field variations occur along the optical axis in the region where the field is produced. Consequently, the non-local field variations will create a great obstacle in aberration correction of the beam.
In the example of the above-cited Japanese application, each polar element is fabricated by inserting the mounting rod into the beam tube and then connecting the polepiece to the front end of the mounting rod. In consequence, much labor is necessary to shape the formed polar element into desired form and dimensions. Furthermore, where the desired shape cannot be achieved due to the condition in which the mounting rod and polepiece are connected, a separate processing step is required.